Shock tube screen choke



June 1967 J. M. STEPHENSON ETAL 3,

SHOCK TUBE SCREEN CHOKE Filed May 20, 1964 coH/v M. srepuswsow .Do/wmza .S'. [@4605 C19,, flaasnr 5. 6714 15 INVENTORS.

United States Patent Office 3,326,033 Patented June 20, 1967 3,326,033 SHOCK TUBE SCREEN CHOKE John M. Stephenson, Oxnard, Donald S. Teague, Jr., Santa Paula, and Robert S. Chapler, Oxnard, Calif., assignors to the United States of America as represented by the Secretary of the Navy Filed May 20, 1964, Ser. No. 369,040 7 Claims. (CI. 73-12) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon and therefor.

This invention relates generally to shock testing apparatus and particularly pressure chokes used therewith.

Since the advent of the atomic bomb, much time and effort has been expended in studying the blast effects of such bombs and the design of protective shelters which would preserve personnel and their necessary equipment when situated in the blast "area. In such studies, much experience and knowledge has been gained partly through exposing such shelters and equipment to actual atomic bomb blasts and partly through the creation of blast simulators and testing equipment designed to observe such blasts effects under controlled, laboratory-installed conditions.

Borrowing from the generally configuration of small wind-tunnels the shock tube has resulted. This structure may be of considerable length with varying diameters and with rectangular or circular cross-sections. At one end, a means of generating a shock wave is installed while the equipment or material being studied may be placed therein or positioned at the other end. While generally ambient air is used in such shock tubes, other gases may be employed. The shock generating means may be pneumatically or explosively controlled. In the apparatus to be described herein, pneumatic shock generating means will be employed utilizing the controlled release of compressed air. The shock tube used was approximately one foot square in cross section with a length of 38 feet and, during the testing period was anchored to a twenty-four inch I-beam.

When first set up, it was necessary to determine the nature of the shock wave as it proceeded down the tube. A series of pressure measuring cells were installed on the interior walls at selected lengths from the entrance. It was found that when the shock wave starts down the tube, the front is steep, the top nearly flat and the decay fairly rapid, say 100 milliseconds. As the shock travels down the tube, the front gets even steeper, the flat top shortens and the decay becomes more rapid, about milliseconds. The ideal shape is an instantaneous rise to a maximum followed by a long exponential decay as a positive-phase duration. Numerous means were conceived and tested to attain this ideal shape. These means included reducing the volume of the compression chamber; placing baflles at the junction of the compression chamber and shook tube to control the reflected shock; throttling the escape of the compressed air into the tube so that more time is taken for all of the air to leave the compression chamber; lengthening the shock tube; and providing a choke at the end of the tube. Of all these and the various other methods employed to improve the wave shape and duration, only considerable lengthening of the tube itself and the use of the choke construction showed promising results. Due to lack of space and the considerable expense involved, the lengthening of the shock tube did not appear practical, particularly as the improvement demonstrated was minor and did not compare favorably with the results produced by the choke.

The principal object of our invention, therefore, is to improve the shape and duration of a shock wave traveling in a shock tube of reasonable dimensions.

The second object of our invention is to accomplish the aforementioned principal object in an inexpensive, yet positive, manner by providing a choke to be installed at the end of shock tube.

Other objects and advantages will be apparent from the following detailed description of the invention, when considered in conjunction with the accompanying drawings wherein:

FIGURE 1 illustrates the general set up of a shock tube apparatus;

FIG. 2 is a perspective elevation of our invention; and

FIG, 3 is a cross-sectional elevation of our invention, taken on the line 3-3, as installed on the outboard end of a shock tube.

With reference to FIG. 1, the shock tube 10 is square in cross-section and may be constructed of a number of sections 11 suitably joined together. The outboard end section 12 may be considered as the model section in which the models being exposed to blast may be placed. Alternatively, a receptacle 13 may be provided for receipt of the model being tested which receptacle may open into the model section 12 through either a clear or restricted opening (not shown).

Compressed air chamber 15 is connected to the inboard end of the shock tube 10 by means of restricted connection 16 which connection includes a diaphragm receiving portion 17. This diaphragm (not shown) may be of plastic or other desired frangible material designed to puncture at a desired pressure or which may be punctured by other pressure controlled means. While not shown, it may be understood that the compressed air chamber 15 may be connected to a source of compressed air of suitable pressure.

The shock tube 10 is shown supported on a large I beam 18. The latter and the compressed air chamber 15 may be supported on levelled ground or other suitable platform 19.

Our shock tube choke 20 is shown installed at the outboard or left end of the tube 10. With reference now to FIGS. 2 and 3, the choke comprises a frame 21 which may be bolted or otherwise suitably secured to the end of the shock tube. Protective wire mesh 22 covers the outside portion of the frame 21 and may be welded or suitably secured thereto. Inboard of the wire mesh, a plurality of layers of perforated or expanded sheets 23 are suspended without the need for any special pre-arrangement, that is, randomly arranged and suspended by means of the bolts 24 which hold frame 21 and sheets 23 against angle irons 25 which latter are secured to the plates of tube model section 12. While the preferred form of perforated or expanded sheets is presently expanded aluminum lath, it is to be understood that these foraminous sheets may be of any suitable material.

In the operation of our device, the compressed air chamber 15 is filled with compressed air to a certain desired pressure or until the bursting pressure of the frangible diaphragm is reached. Generally, pressures from 50 to p.s.i. are used. In the former event, the diaphragm may be ruptured by mechanical means, manually operated, or as a function of pressure in the chamber. Upon the rupture of the diaphragm, the shock wave is introduced into the shock tube; the wave which quickly propagates to the choke 20 will be partially and progressively reflected at each of the layers of perforated or expanded sheets 23 causing an increase in the period of decay or duration of the wave and an increase in overpressure. Whereas prior to the use of our choke, the overpressures were very small and the positive phase duration lasted but approximately 40 milliseconds, the use of our inven- J tion increased the overpressures to some 12 p.s.i. and increased the positive phase duration up to 400 milliseconds.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that Within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. In a shock tube having a propagating shock wave of pressure introduced at one end and facilities between said one end and a discharge end for subjecting models to said shock wave at a point remote from said one end, perforated means extending across the discharge end of said shock tube for increasing the overpressures and the periods of decay of said shock wave of pressure.

2. A combination as claimed in claim 1 comprising a plurality of foraminous sheet material secured to the discharge end of said shock tube.

3. A combination as claimed in claim 1 comprising an open frame secured to the discharge end of said shock tube, a protective wire mesh and a lurality of sheets of expanded metal secured across said open frame.

4. In a shock tube having a propagating shock wave of pressure introduced at one end and subjecting models to said propagating shock wave at a point within the tube closely spaced from the other end,

perforated means attached to the tube extending thereacross substantially perpendicular to the path of travel of said shock wave for increasing the period of decay and increasing the overpressure of said shock wave, whereby the increased duration and overpressure of the shock Wave simulates a shock wave created by a nuclear explosion.

5. In a shock tube as claimed in claim 4 wherein the means for increasing duration and overpressure comprises:

a plurality of layers of perforated sheets attached to said tube.

6. In a shock tube as claimed in claim 4 wherein:

the means for increasing duration and overpressure is mounted at the other end of the shock tube; and

said means extends across substantially the entire other end.

7. In a shock tube as claimed in claim 4 wherein the perforated means comprises:

a plurality of layers of perforated sheets attached to said tube; and

said plurality of sheets being randomly arranged such that said propagating shock wave is partially and progressively reflected by each of said plurality of sheets.

References Cited UNITED STATES PATENTS 2,570,129 10/1951 Johnson 73147 2,636,572 4/1953 Fleisher -489 XR 2,691,761 10/1954 Smith 73167 2,720,276 10/1955 Droeger 18146 2,805,571 9/1957 Graham 73-147 3,130,575 4/1964 Rogers 73-12 3,204,395 9/1965 Howard 55-505 3,236,263 2/1966 Holderer 73147 XR RICHARD C. QUEISSER, Primary Examiner. I. J. SMITH, I. I. GILL, Assistant Examiners. 

1. IN A SHOCK TUBE HAVING A PROPAGATING SHOCK WAVE OF PRESSURE INTRODUCED AT ONE END AND FACILITIES BETWEEN SAID ONE END AND A DISCHARGE END FOR SUBJECTING MODELS TO SAID SHOCK WAVE AT A POINT REMOTE FROM SAID ONE END, PERFORATED MEANS EXTENDING ACROSS THE DISCHARGE END OF SAID SHOCK TUBE FOR INCREASING THE OVERPRESSURES AND THE PERIODS OF DECAY OF SAID SHOCK WAVE OF PRESSURE. 